Between the two World Wars Knox worked on the great commentary on Herodas that had been started by Headlam, damaging his eyesight while studying the British Museum's collection of papyrus fragments, but finally managing to decipher the text of the Herodas papyri. The Knox-Headlam edition of Herodas finally appeared in 1922.[10]

During World War I he had been elected Librarian at King's College, but never took up the appointment. After the war Knox intended to resume his research at King’s, but was persuaded by his wife to remain at his secret work; indeed, so secret was this work that his own children had no idea, until many years after his death, what he did for a living, and his contribution to the war effort.[8]

The Enigma machine became available commercially in the 1920s. In Vienna in 1925,[13] Dilly bought the Enigma 'C' machine evaluated by Hugh Foss in 1927 on behalf of GC&CS. Foss found "a high degree of security" but wrote a secret paper describing how to attack the machine if cribs — short sections of plain text — could be guessed.[3] When — a decade later — Knox picked up this work, he developed a more effective algebraic system (rodding) based on the principles described by Foss.[13]

Germany Navy adopted Enigma in 1926, adding a plug-board ('stecker') to improve security. Nazi Germany supplied non-steckered machines to Franco'sNationalists in the Spanish Civil War. On 24 April 1937, Knox broke Franco's Enigma[3] but knowledge of this breakthrough was not shared with the Republicans.[14] Soon afterwards, Dilly began to attack signals between Spain and Germany encrypted using steckered Enigma machines.[3]

GC&CS began to discuss Enigma with France's Deuxième Bureau in 1938, obtaining from the Bureau details of Wehrmacht Engima supplied by Asché and signal intercepts, some of which must have been made in Eastern Europe. This led the French to disclose their links with Polish cryptographers.[3] As GS&CS chief cryptographer[2] Dilly, together with Hugh Foss and Alastair Denniston, represented GC&CS at the first Polish-French-British meeting at Paris in January 1939. The Poles were ordered to disclose nothing of importance at this time, and the British codebreakers left disappointed. Dilly described his system of rodding, and he left the Polish codebreakers sufficiently impressed for his presence to be requested at a second meeting.[3]

Knox grasped everything very quickly, almost quick as lightning. It was evident that the British had been really working on Enigma ... So they didn't require explanations. They were specialists of a different kind, of a different class.

After the meeting, he sent the Polish cryptologists a very gracious note in Polish, on official British government stationery, thanking them for their assistance and sending "sincere thanks for your cooperation and patience".[2] Enclosed were a beautiful scarf featuring a picture of a Derby winner and a set of paper 'batons'.[15]

I don't know how Knox's method was supposed to work, most likely he had hoped to vanquish Enigma with the batons. Unfortunately we beat him to it.

Alan Turing worked on Enigma during the months leading to the outbreak of World War II in September 1939, and occasionally visited GC&CS's London HQ to discuss this problem with Knox. By November 1939, Turing had completed the design of the bombe — a radical improvement of the Polish bomba.[16]

To break non-steckered Enigma machines (those without a plugboard), Knox (building on earlier research by Hugh Foss[3]) developed a system known as 'rodding', a linguistic as opposed to mathematical way of breaking codes. This technique worked on the Enigma used by the Italian Navy[17] and the German Abwehr. Knox worked in 'the Cottage', next door to the Bletchley Park mansion, as head of a research section, which contributed significantly to cryptanalysis of the Enigma.[6]

In October 1941, Dilly solved the Abwehr Enigma.[4] Intelligence Services Knox (ISK) was established to decrypt Abwehr communications.[4] In early 1942, with Knox seriously ill, Peter Twinn took change of running ISK[5] and was appointed head after Knox's death.[4] By the end of the war, ISK had decrypted and disseminated 140,800 messages.[4]

Knox's work was cut short when he fell ill with lymphoma.[18] When he became unable to travel to Bletchley Park, he continued his cryptographic work from his home in Hughenden, Buckinghamshire, where he received the CMG.[19] He died on 27 February 1943.[19] A biography of Knox, written by Mavis Batey, one of 'Dilly's girls', the female codebreakers who worked with him, was published in September 2009.[20]

^Keeley 2008 States "Professor Denis Smyth, of the University of Toronto, an expert on Second World War intelligence operations, said that the British codebreaker Alfred Dilwyn Knox cracked the code of Franco's machine in 1937, but 'this information was not passed on to the Republicans'."

Andrew, Christopher (2011). "Chapter 1: Bletchley Park in Pre-War Perspective". In Erskine, Ralph; Smith, Michael. The Bletchley Park Codebreakers. Biteback Publishing. pp. 1–12. ISBN978-1849540780. (Updated and extended version of Action This Day: From Breaking of the Enigma Code to the Birth of the Modern Computer Bantam Press 2001)

Foss, Hugh (2011). "Chapter 3: Reminiscences on the Enigma". In Erskine, Ralph; Smith, Michael. The Bletchley Park Codebreakers. Biteback Publishing. pp. 35–39. ISBN978-1849540780. (Updated and extended version of Action This Day: From Breaking of the Enigma Code to the Birth of the Modern Computer Bantam Press 2001)

Copeland, Jack (2011). "Chapter 19: Colossus and the Dawning of the Computer Age". In Erskine, Ralph; Smith, Michael. The Bletchley Park Codebreakers. Biteback Publishing. pp. 305–327. ISBN978-1849540780. (Updated and extended version of Action This Day: From Breaking of the Enigma Code to the Birth of the Modern Computer Bantam Press 2001)

Kozaczuk, Władysław (1984), Enigma: How the German Machine Cipher was Broken, and how it was Read by the Allies in World War Two - Edited and translated by Christopher Kasparek, Frederick, Maryland (a substantially revised and augmented translation of W kręgu enigmy, Warsaw, Książka i Wiedza, 1979, supplemented with additional appendices by Marian Rejewski.) (2 ed.), University Publications of America, ISBN978-0-89093-547-7